8 PREFACE. 



of the one's becoming a trout and of the other's becom- 

 ing a salmon? 



The fact is, we can make no show of accounting for 

 the complex phenomena of heredity with mere material 

 units ; we can never reach these phenomena from 

 below, but must begin farther up and make the as- 

 sumption of vital units and hereditary units, if there is 

 to be any advance in this field. 



It is undoubtedly a splendid aim which the newly 

 founded science of developmental mechanics has set 

 itself of laying bare the entire causal line leading from 

 the egg to the finished organism ; yet, however much 

 we may wish to see the success of this plan realised, 

 we cannot disguise the fact that little or nothing is 

 to be accomplished by it in the settlement of the 

 problems of heredity. It is impossible to suspend the 

 study of heredity until this mechanics is completed, 

 and even if we could it would help us little, for the 

 riddles of heredity are not concealed in the ontogenesis 

 of types, or, to give an example, in the developmental 

 history of man as a race, but in the ontogenesis of 

 individuals, in that of a definite and particular man. 

 This last ontogenesis exhibits the phenomena of varia- 

 tion, of reversion, of the predominance of the one or 

 the other parent, etc., and no one is likely to believe 

 that inductive evolutional inquiry alone will ever afford 

 us knowledge of these minute and delicate processes, 

 which, in their bearing on the total resultant devel- 

 opment, phylogenesis, are after all the most important 

 of all. 



There is, accordingly, no choice left. If we are 

 really bent on scientifically investigating the question 

 of heredity, we are obliged perforce to form from the 

 observed facts of heredity a highly detailed and elab- 



